**3. Resistance detection methods**

Several standard methods are there to detect resistance development. At the field level, resistance development is suspected when control failure occurs. There are various reasons for the control failure of pests at the field level. Pesticide resistance is one of the important reasons for control failure. Once control failure is observed in the field, the population is collected and tested in the laboratory for confirmation. In the laboratory leaf-dip bioassays, biochemical assays, or molecular assays are conducted to know the susceptibility status of the population.

#### **3.1 Leaf-dip bioassays**

Several bioassay procedures are there to detect the resistance status but the leafdip bioassays are usually followed in the case of whitefly. In leaf-dip bioassay serial dilution of the tested insecticides are prepared from the commercial formulation of insecticide. Leaves are collected from the unsprayed field and washed under tap water. Then the washed leaves are air-dried under normal room temperature. Leaf discs of appropriate diameter are prepared and dipped in the insecticidal solution and agitated slightly for 5 to 10 seconds for complete wetting. Sometimes whole leaves are dipped in the insecticidal solution. Then the treated leaves or discs are dried for a few minutes until the surface liquid gets dried. In case of control, leaves

or discs are dipped in distilled water. The leaves are kept on a Petri plate or in an appropriate container with a perforated lid or covered with a muslin cloth. Then 20 to 30 adult whiteflies are released in the Petri plates. The tests were carried out and maintained at 24 to 25°C temperature. Mortality is observed 48–72 hours after the treatment depending on the type of insecticides tested. Adults showing no signs of movement are considered as dead. Three replicates are usually carried out for each concentration of each insecticide and also for controls. Percentage of mortality of nymph for each concentration of test insecticide and control was calculated using the following formula:

$$\text{Percent mortality} = \frac{\text{Number of dead whiteflows}}{\text{Total number of white files treated}} \times 100\tag{1}$$

Then corrected percent mortality was calculated using Abbott's formula [42] based on the control mortality, if any. The corrected mortality data of each test insecticide of each location were subjected to probit analysis based on Finney [43]. Now, the resistance ratio (RR) or resistance factor (RF) is calculated using the following formula:

$$\text{Resistance Ratio (RR)} = \frac{\text{LC50 of a field population}}{\text{LC50 of the most susceptible population}} \tag{2}$$

#### **3.2 Biochemical assays**

Biochemical assays are frequently used to characterize resistance mechanisms but they can also be used to detect resistance status. In biochemical assays, the activity of detoxifying enzymes *viz.,* general esterases (GEs), glutathione S-transferases (GSTs), and cytochrome P-450 dependent mixed-function oxidases are estimated using spectrophotometer or microplate reader and compared in different populations. In resistant populations, the activity of pesticide-degrading enzymes is overproduced resulting in efficient degradation of the pesticides.

#### **3.3 Molecular assays**

Molecular assays detect genes or mutations involved in resistance. There are different types of nucleic acid-based assays for resistance study. Quantitative PCR, reverse transcription plus quantitative PCR, whole transcriptome sequencing, etc., are some of the important molecular assay techniques. These techniques require highly sophisticated and costly instruments and reagents.

### **4. Geographical distribution of insecticide resistance in whitefly**

The whitefly has developed multifold resistance against various groups of insecticides. Approximately 650 cases of insecticide resistance have been reported in the genus *Bemisia,* and it has developed resistance against more than 60 active ingredients [44]. Among two biotypes, MED (biotype Q) is considered to be more resistant to insecticides than MEAM1 (biotype B) [28, 35, 45, 46]. In the case of India, the species Asia II-7 has developed more resistance to insecticides than Asia I and Asia II-1 (**Table 1**) [30].


*Insecticide Resistance in Whiteflies* Bemisia tabaci *(Gennadius): Current Global Status DOI: http://dx.doi.org/10.5772/intechopen.101954*

#### *Insecticides - Impact and Benefits of Its Use for Humanity*



#### *Insecticide Resistance in Whiteflies* Bemisia tabaci *(Gennadius): Current Global Status DOI: http://dx.doi.org/10.5772/intechopen.101954*

#### **Table 1.**

*Status of insecticide resistance to different classes of insecticides in whitefly.*

### **5. Consequence of insecticide resistance**

Control failure of a pest is a very common phenomenon in the case of resistance development. As a result of the control failure, farmers usually increase the frequency of application along with the higher dose of the chemical. Sometimes farmers reduce the interval between two consecutive sprays. All these incidences ultimately worsen the resistance scenario. When the chemical is no more effective to control the pest, then the ineffective pesticide is replaced with a new one provided that a suitable one is available. Initially, the new one also gives good efficacy. However, after repeated and prolonged use, the new one also becomes ineffective. This cycle continues and the pests develop multiple resistance. This scenario has been repeated in various agroecosystems of the world and the effect is enormous. Moreover, under the present scenario, it will be increasingly difficult to design, develop, and introduce new pesticides to solve the problems. Even there is a chance of development of resistance prior to the introduction of a new chemical. Hence, every care should be taken to delay and combat the resistance issue.

The development of pesticide resistance leads to control failure of the pests and thereby increases the cost of production due to higher requirements of the chemicals and frequent application costs. Control failure resulting from the resistance development leads to more use of the chemicals that ultimately deteriorates the quality of the produces. The pesticide residues in the product may be harmful to the health of the consumers and thereby society as a whole. In addition to that pest, resurgence may occur due to disruption of the pest-defender ratio. The secondary pests may attain the status of major pest due to harmful effects on the available natural enemies. As whitefly acts as a vector of various virus diseases of plants, the viral diseases of the plants may increase that may result in huge economic loss to the farmers.

#### **6. Integrated resistance management (IRM) strategy**

The biological characteristics like migratory ability and polyphagous nature of whitefly promote resistance development. These characteristics cannot be controlled directly. Therefore, multipronged strategies need to be adopted to manage the problem**.** Insecticide resistance management (IRM) strategies need to be followed to delay and combat the problem. Excessive use of chemical insecticide for the management of whitefly is the main cause of insecticide resistance development. So, we need to use insecticide as last resort. The frequency of insecticide use and thereby the degree of selection pressure is the main driving force for the development of insecticide resistance. Emphasis has to be given in the rotation of insecticides having different modes of action (MOA) based on IRAC's MoA classification scheme [69]. Insecticides having similar modes of action should not be used frequently. Moreover, the information on the cross-resistance phenomenon needs to be considered. The chance of multiple resistance development is also there in case of extreme selection pressure [50]. Chemicals having different modes of action are enlisted in **Table 2**. These insecticides may be used in rotation programs for the management of whitefly. The availability and use of an insecticide in a specific crop vary in different countries and it is regulated by law. Hence, it is important to check whether an insecticide is approved for use in a particular crop or not before recommending it. The mode of action, dose, and waiting period of chemicals that are approved for management of whitefly in India are as follows [70]. The crossresistance information needs to be taken into account before recommendation. The insecticides should be used as per the label claim and should be selected based on the local recommendation or local efficacy and selectivity.


*Insecticide Resistance in Whiteflies* Bemisia tabaci *(Gennadius): Current Global Status DOI: http://dx.doi.org/10.5772/intechopen.101954*


#### **Table 2.**

*Insecticides with different modes of action for rotation program in whitefly management.*

Spraying of mixture formulation of insecticides is also an important tactic for the management of whitefly. The basis of this approach is that individual insecticides in the mixture formulation have different modes of action and they lack crossresistance [71, 72]. However, it has been found that the frequent and injudicious use of synergized insecticides lead to the development of high degree of resistance to both the chemicals. Despite several controversies over the use of mixture formulation of insecticide, the insecticide mixtures are still popular among farmers.

The chemical insecticide should be used as a last resort for the management of whitefly. Insecticides' use may be regulated in such a way that the full diversity of available chemicals is exploited instead of over-reliance on a single chemical for a long time. The Israeli strategy introduced in 1987 to preserve susceptibility to insecticides by optimizing and restricting their use to a single treatment per year [40, 41, 72] may be followed. The use of broad-spectrum insecticides should be

#### *Insecticide Resistance in Whiteflies* Bemisia tabaci *(Gennadius): Current Global Status DOI: http://dx.doi.org/10.5772/intechopen.101954*

avoided to minimize nontarget toxicity. This will conserve the natural enemies. The status of resistance should be monitored at regular intervals.

IPM strategies must be emphasized to combat the resistance problem. Some of the IPM strategies for management of whitefly are mass trapping and monitoring of whitefly using yellow sticky traps, use of entomopathogenic fungi (EPF), augmentative release of natural enemies, etc.
